1. Field of the Invention
This invention relates, in general, to offshore oil well risers that convey petroleum from producing wells on the sea floor to a floating platform on the sea surface, and in particular, to risers that are tensioned at their bottom ends to enable them to accommodate large motions of the platform relative to the wells without sustaining damage.
2. Description of Related Art
Conventional “dry tree” offshore floating petroleum production platforms include such “low heave” platforms as Spars, Tension Leg Platforms (“TLPs”), and Deep Draft semi submersible platforms. These platforms are capable of supporting a plurality of vertical production and/or drilling risers. These platforms typically comprise a well deck, where the surface, or dry, trees, which are mounted on top of the risers, are located, and a production deck where crude oil from one or more sub-sea wells is collected in a manifold and conveyed to a processing facility to separate the oil from entrained water and gas. In conventional dry tree offshore platforms, each of the vertical risers extending from the well heads to the well deck are supported thereon by a tensioning apparatus, and hence, are referred to as Top Tensioned Risers (“TTRs”).
One type of conventional TTR system uses active hydraulic tensioners connected to the well deck of the offshore platform to support each riser independently of the others. See, e.g., U.S. Pat. No. 6,431,284 to L. D. Finn et al, and FIG. 1 of the appended drawings. Each riser 100 extends vertically from a well head 102 on the sea floor to a well deck 104 of the platform, and is supported thereon by hydraulic cylinders 106, such that the platform can move up and down relative to the risers and thereby partially isolate the risers from the heave motions of the platform. A surface tree 108 is connected on top of the riser, and a high pressure, flexible jumper 110, typically incorporating an elastomer, connects the surface tree to the production deck 112. However, as tension and stroke requirements of the active tensioners increases, they become prohibitively expensive to deploy. Furthermore, the offshore platform must be capable of supporting the entire load of the risers, which can be substantial.
Another known TTR system (see, e.g., U.S. Pat. No. 4,702,321 to E. E. Horton and FIG. 2 hereof) uses passive “buoyancy cans” 202 to support a riser 204 independently of the floating platform. In this system, each riser extends up vertically from a well head 206 through the keel of the platform and to the well deck 208 of the platform, where it connects to a “stem” pipe 210, to which the buoyancy cans are attached. The stem extends above the buoyancy cans and supports the work platform to which the riser and its associated surface tree are attached. A high pressure, flexible jumper 212 connects the surface tree 214 to the production deck 216. As the risers are independently supported by the buoyancy cans relative to the platform's hull, the hull can move up and down relative to the risers, and the risers are thereby isolated from the heave motions of the platform. However, the buoyancy cans must provide sufficient buoyancy to provide the required top tension in the risers, and to support the weight of the can, the stem and the surface tree. In deeper waters, the buoyancy required to provide this support is substantially greater, requiring larger buoyancy cans. Consequently, the deck space required to accommodate all the risers also increases. Manufacturing and deploying individual buoyancy cans for each riser is also costly.
In both of the above TTR systems, the tension applied to the riser must be sufficient not only to support the weight of the riser system, but also to ensure that the riser does not go slack or vibrate in response to current vortices. In general, the required top tension will be in the range of from about 1.4 to 1.6 times the weight of the riser system. This requirement dramatically increases the cost of the tensioning system, and in some deepwater applications, where the weight of the riser is substantially greater, can result in an overstress of the risers.
A third type of dry tree riser system comprises the so-called “riser tower,” such as that described in U.S. Pat. No. 6,082,391 to F. Thiebaud et al and illustrated in FIG. 3. In this system, the riser tower includes one or more rigid vertical pipes 302 connected to the seafloor through a pivot connection or a stress joint 304. The pipes are supported by a large top buoyancy device 306, which provides sufficient buoyancy to support the pipes and prevent them from going slack or vibrating in response to sea currents. Flexible jumpers 308 are used to connect the vertical pipes to a floating support 310. This type of riser system is both expensive and difficult to deploy.
Conventional “wet tree” offshore platforms include Floating Production Storage and Off-loading (“FPSO”) and semi submersible platforms, both of which have relatively greater heave responses. The relatively larger motions experienced by these types of platforms make the support of vertical drilling and production risers impractical. These types of platforms are generally used in connection with a sub-sea “completion system,” i.e., sub-sea trees which are connected to wells arranged on the seafloor. Produced crude oil may be carried along the seafloor with “flow lines” and collected in a manifold. Production risers convey the crude oil from the manifold or sub-sea trees to the process equipment of the floating support platform. As the support platform experiences relatively large motions, both heave and horizontal, the production risers must be designed to withstand these greater motions.
Wet tree riser systems can comprise flexible, e.g., elastomeric, risers. As shown in FIG. 4, flexible risers 402 are directly connected to a floating platform 404 and present a catenary shape from the floating support down to the sea floor, such as those shown connected to the FPSO platform 404 illustrated in FIG. 4. They are able to accommodate relatively large platform motions due to their flexibility. However they are both heavy and expensive. Alternatively, the risers can comprise so-called Steel Catenary Risers (“SCRs”). Steel Catenary risers are made primarily of steel and connect directly to the floating support by means of a flexible joint or similar arrangement, and like the flexible risers, present a catenary shape when deployed. Additionally, since they are made of steel, SCRs are less expensive. However, due to their greater stiffness, they are prone to fatigue problem resulting from the dynamic motions that they must undergo, and may require relatively greater lengths to accommodate the motions of the platform satisfactorily.
In the above prior art riser systems, the risers are either vertical and supported by a tensioning system independent of the floating platform, wherein a flexible jumper is used at the top of the vertical riser to absorb the relative motion between the vertical riser and the floating platform, or they are supported directly by the floating platform and present a catenary shape requiring a relatively longer length of pipe to absorb the motions of the floating platform. Thus, in the former types of systems, the platform motions are absorbed by the upper part of the riser, and therefore require a critical degree of top tension to prevent a destructive compression of the risers and the occurrence of riser collisions, and in the latter types of the systems, the risers must sag to absorb motions, and therefore require substantially great lengths of pipe to function.
In light of the foregoing drawbacks of the prior art riser systems, a long felt but as yet unsatisfied need exists in the petroleum industry for a simple, low-cost, yet safe and reliable off-shore oil well riser system that compensates for the motions of an associated floating platform.